Silicon carbide (SiC) is a wide band gap semiconductor having a broad forbidden band width and having properties far surpassing conventional silicon (Si) in terms of voltage resistance, heat resistance, etc., and research and development thereof is a next-generation semiconductor are advancing.
One of the techniques for growing a silicon carbide single crystal (SiC single crystal) is a physical vapor transport (PVT) method. More specifically, in this method, which is also called an Modified-Lely method, a seed crystal composed of SiC is attached to the lid body of a crucible and after placing an SiC raw material in the container body of the crucible, the SiC raw material is sublimated to grow a bulk SiC single crystal on the seed crystal. At this time, an impurity can be doped into the single crystal under growing and, for example, in the case of an n-type SiC single crystal, a nitrogen (N2) gas can be added to the atmosphere gas during growth. A bulk SiC single crystal (ingot) having a substantially columnar shape is obtained and then cut out in general to a thickness of approximately from 300 to 600 μm, and thereafter, an SiC single crystal substrate is produced and used for the manufacture of an SiC device in the fields of power electronics, etc.
The crystal growth by this PVD method requires a temperature in excess of 2,000° C. and moreover, since the crystal growth is per formed by providing a temperature gradient in the crucible where a seed crystal and an SiC raw material are placed, crystal defects such as dislocation defects and stacking faults are anyhow contained in the obtained SiC single crystal. Of these, dislocation defects include threading edge dislocation, basal plane dislocation, and screw dislocation. For example, it has been reported that a commercially available SiC single crystal substrate has approximately from 8×102 to 3×103 (/cm2) screw dislocations, from 5×103 to 2×104 (/cm2) threading edge dislocations, and from 2×103 to 2×104 (/cm2) basal plane dislocations (see Non-Patent Document 1).
In recent years, research and investigation relating to the crystal defects of SiC and the device performance have advanced, and the effects of various defects are becoming clear. Among others, screw dislocation has been reported, for example, to cause a leakage current in a device or decrease the life of a gate oxide film (see Non-Patent Documents 2 and 3). In order to manufacture a high-performance SiC device, an SiC single crystal substrate having less screw dislocations is at least required.
Accordingly, Patent Document 1 discloses a method for producing an SiC single crystal, wherein a bulk SiC single crystal is grown by assigning the growth plane to a plane with an offset angle (off angle) of 60° or less from a {0001} plane and using a dislocation-controlled seed crystal having, in a region corresponding to 50% or less of the growth plane, a screw dislocation-generatable region capable of generating screw dislocations with a higher density than at the periphery of an SiC single crystal under growing and at the time of growth, a silicon carbide single crystal is grown such that a region formed by projecting the screw dislocation-generatable region in the c-axis direction overlaps with the c-plane facet. In Patent Document 1, it is disclosed that by this production method, as SiC single crystal having a region with a high screw dislocation density and a region with a lower screw dislocation density than the region above can be manufactured.
However, in this production method, c-plane growth for growing the crystal in the c-axis direction and a-plane growth for growing the crystal in a direction perpendicular thereto must be performed so as to obtain the dislocation-controlled seed crystal. Moreover, in order to obtain an SiC single crystal having many regions with a low screw dislocation density according to this method, after preparing a dislocation-controlled seed crystal more reduced in the screw dislocation-generatable region by repeating those c-plane growth and a-plane growth, an SiC single crystal needs to be grown. Thus, this production method has a problem with productivity.
Patent Document 2 discloses a method for producing an SiC single crystal, including a first growth step of growing a silicon carbide single crystal to a thickness of at least 0.5 mm at a first growth atmosphere pressure of 3.9 to 39.9 kPa and a first growth temperature with the temperature of the seed crystal being from 2,100° C. to less than 2,300° C., and a second growth step of growing the silicon carbide single crystal to a larger thickness than in the first growth step at a second growth atmosphere pressure of 0.13 to 2.6 kPa and a second growth temperature with line temperature of the seed crystal being higher than the first growth temperature and being less than 2,400° C. Patent Document 2 discloses a method for obtaining a silicon carbide single crystal substrate having less screw dislocations in the peripheral part than in the central part of the substrate, which is cut out from a bulk silicon carbide single crystal grown by the above-described method for producing an SiC single crystal.
According to this method, in the first growth step, the screw dislocation in the SiC single crystal is structurally converted to a stacking fault. In particular, such a structural conversion is likely to occur in the peripheral part compared with the central part of the growth surface in the process of growing of an SiC single crystal, and the screw dislocation density in the peripheral part can be reduced to about one-tenth relative to the central part of the substrate. This method is therefore very effective as a method for decreasing the number of screw dislocations. However, the region in which the number of screw dislocations decreases is a doughnut-like peripheral region excluding the central part of the substrate, and there is room for studies from the viewpoint of more increasing the device yield.
In connection with the method of Patent Document 1, Patent Document 3 discloses a method for producing an SiC single crystal ingot, in which a (0001) facet plane is formed in the edge part of an SiC single crystal ingot by using, as a seed crystal, a base substrate having an off angle of 0.1 to 10° in the <11-20> direction (or <1-100> direction) with respect to (0001) plane. In Patent Document 3, it is disclosed that since nitrogen is readily captured in the portion below the surface having formed thereon the facet plane, a region having a relatively low nitrogen concentration is formed on the center side of the SiC single crystal ingot and an SiC single crystal substrate having a reduced variation in the nitrogen concentration is obtained. According to this method, the number of dislocations is supposed to be decreased substantially in the entire region of the obtained ingot, but the detailed mechanism of decreasing the number of dislocations is unclear, and in addition, although it is supposed that the etch pit density of the obtained SiC single crystal could be reduced to ½ to 1/20 of that (1×104 to 5×104 cm−2) of the base substrate, how the dislocations are practically distributed in the substrate is unknown.
Patent Document 4 discloses a production method including a first growth step of growing a silicon carbide single crystal at a first growth atmosphere pressure of 3.9 to 39.9 kPa and a first growth temperature with the temperature of the seed crystal being from 2,100° C. to less than 2,300° C., and a second growth step of growing the silicon carbide single crystal to a larger thickness than in the first growth step at a second growth atmosphere pressure of 0.13 to 2.6 kPa and a second growth temperature with the temperature of the seed crystal being higher than the first growth temperature and being less than 2,400° C. In Patent Document 4, it is disclosed that the screw dislocation is structurally converted to a stacking fault in the first growth step, and the temperature of the seed crystal is raised in the second growth step, whereby a high-speed growth can be performed with good productivity while obtaining a high-quality silicon carbide single crystal.
Patent Document 5 discloses a production method of performing the crystal growth of a silicon carbide single crystal in the slate of an impurity being added so as to control the volume resistivity, by using a seed crystal in which the crystal growth plane has an offset angle of 2 to 15° from a {0001} plane. In Patent Document 5, it is disclosed that when an SiC single crystal substrate cut out from such a crystal is used, a high-performance semiconductor device of an extremely small electrical power loss can be manufactured with good yield.
Patent Document 6 discloses a method for producing an epitaxial silicon carbide single crystal substrate, including forming, on the substrate, a plurality of suppression layers having different nitrogen concentrations for controlling the basal plane dislocation density, and forming, on the suppression layer, an active layer of a silicon carbide single crystal thin film. In Patent Document 6, it is disclosed that by stepwise changing the nitrogen concentration, an appropriate crystal strain not newly causing a crystal dislocation can to produced at the interface between respective suppression layers or at the interface between a suppression layer and an active layer, making it possible to concentrate the strain at the interface, and this effectively acts to suppress the occurrence of basal plane dislocation.
Patent Document 7 discloses a method wherein an SiC single crystal boule is grown by sublimation on an SiC single crystal seed while making a change in the temperature, a change in the temperature gradient, and a change in the composition and pressure of atmosphere gas, and a threading dislocation during growth of an SiC single crystal boule is thereby converted to a basal plane dislocation to allow the threading dislocation density of the grown SiC single crystal boule to substantially decrease as it proceeds from the initially grown boule to the finally grown boule. In Patent Document 7, it is disclosed that propagation of a threading dislocation during growth from the seed to the grown crystal is minimized by this method.
However, none of Patent Documents 1 to 7 discloses that a method for producing a silicon carbide single crystal wherein a screw dislocation-reduced region is ensured in a wide range by efficiently reducing occurrence of screw dislocation while spiral growth centering on screw dislocation is utilized. In addition, it is neither disclosed nor suggested in any of Patent Documents 1 to 7 that the nitrogen partial pressure in the growth atmosphere and the step supply from the facet affect the reduction in the screw dislocation density